This invention relates to novel compositions for stabilizing halogenated hydrocarbon polymers against thermal degradation.
Halogen containing vinyl resins are widely marketed today, having achieved an outstanding position of prominence in the field of synthetic polymers. However, in many cases, such resins are prone to undergo thermal degradation and discoloration during customary processing steps involving elevated temperatures. Thus, such major commercial resins as those based upon homopolymers or copolymers of vinyl chloride and/or vinylidene chloride usually discolor noticeably even during normal melt processing and fluxing steps such as milling, compounding, molding, extrusion, etc.
The severity of the discoloration problem will, of course, vary from resin to resin, with the particular method of processing used and the associated temperature and thermal exposures involved. However, in order to assure saleable finished products of good color and/or clarity, it is common practice to blend with such halogen containing polymers one or more stabilizing additives which inhibit or control degradation and discoloration during processing and fabrication operations.
Over the years many different stabilizers have been introduced and considerable progress has been made in protecting halide resins against thermal degradation.
Some of the more important heat stabilizers which have been used commercially in this field are the alkali and alkaline earth metal salts of various acids or partially esterified acids, several inorganic salts and various organoderivatives of tin, lead and antimony, as well as certain non-metallic compounds such as organic phosphites, epoxy derivatives, polyols, phenolic compounds, and various compounds of sulfur and/or nitrogen. Specific prior art additives which have been used in stabilizing vinyl halide resins are described and discussed in considerable detail in many technical treatises such as the book, "The Stabilization of Polyvinyl Chloride" by F. Chevassus and R. deBroutelles (publ. 1963 by St. Martin's Press, N.Y., N.Y.).
However, in spite of the progress which has been made and the multiplicity of stabilizing additives already marketed, discoloration is still a practical problem which detracts from the realization of the full potential of halide resins. Thus, even the best stabilizers, at tolerable use levels, can only reduce or delay discoloration. Because of this, much effort has been devoted to finding combinations of stabilizers which can provide better overall peformance. Although best current practice generally involves various such multi-component stabilizer systems, a rather delicate balance is generally involved in using same so as to achieve optimized heat stabilization without encountering other problems such as physical incompatability or adverse chemical reactions within the complete resin formulation.
In view of such complications and as confirmed in recent review articles such as the one on "Heat Stabilizers" by S. D. Brilliant on pages 228-234 of "Modern Plastics Encyclopedia" 1974-75, Vol. 51, No. 10A, (publ. Oct. 1974 by McGraw-Hill Co. of N.Y., N.Y.), the selection and use of stabilizers and stabilizer system in formulating halogenated vinyl resin compounds continues to be a very difficult and empirical art. Accordingly, the search goes on for better, safer and/or more dependable stabilizer additives and systems.
The general object of this invention is to provide improved means for protecting halogen containing resins from thermally induced decomposition and discoloration. Another object is to provide new and improved stabilizing additives for such resins. A further object is to provide such additives which are highly effective in low concentrations and useful in protecting a wide variety of halogen containing resins under many different conditions.
Other objects are to provide new and improved stabilizer systems and compatible formulations, as well as vinyl halide resin compounds containing same, which retain excellent color (e.g. clarity or whiteness) during normal high temperature processing and fabrication steps.
Still other objectives and advantages of this invention will become apparent from the detailed description thereof which follows.